16x go to GPU - and this one on A320 and B350/450 can't be split (aka bifurcation), to force MB makers to buy more expensive chipsets for SLI.

4x is left for the chipset (on Intel it is same). Chipset uses them for lot of things, including making multiple PCIe 2.0 lines.

Note Actual Ryzen die has full 32 PCIe 3.0 lines, but it had been decided that it will be too nice to allow consumer PCs to use them all. References that somehow AMD does not have free pins in socket is absolute bullshit.

Note! Ryzen 2200G or 2400G are designed such way that they provide only 8 PCIe 3.0 lines to GPU instead of 16x and they can't be split.
So, forget about idea of using them for any advanced tasks.

Chipset difference

If you look carefully you understand that you must avoid B350/B450 chipsets at all cost, not mentioning A320.

First reason - they do not allow to split x16 into two PCIe x8 slots, this is very bad for us* (see note below).

Second reason - they have less general purpose PCIe 2.0 lines, 2 less sounds like not much, but it is very important on our limited consumer CPU IO budget.

Note about bifurcation/split. As usual in capitalism - it is artificially removed and disabled feature. On Asrock ITX B350/B450 boards such options is present and works. It had been done due to large request from users/buyers and some other companies do this (keeping silence so it won't become open knowledge among too many, yet you still need to modify BIOS to enable SLI). But even with such options it is horrible life as you need custom impossible to find riser and turning GPU card into vertical state or such.

SSD performance limits on Ryzen

Second M.2 slot on almost all boards where it is present will be using PCIe 2.0 x2 or x4 mode.

PCIe 2.0 per-lane throughput is 5GT/s or 500MB/s of actual data transfer per lane. x2 will be capped at 1GB/sec recording/reading and x4 at 2GB/sec recording/reading, no matter if the limit of SSD you put in it is higher.

First M.2 slot uses four dedicated v3.0 lines. Single PCIe 3.0 lane at 8GT/s, can send 985MB/s. This limit is close to 4GB/sec.

Big difference is that now if you took up top M.2 slot with SSD you can split x16 slot and into two x8 ones.

If you do not work with raw 6K/8K it won't be noticeable, otherwise it'll hit performance some.

Now we go into humble low M.2 slot and it's PCIe 2.0 x2 mode.

PCIe 2.0 per-lane throughput is 5GT/s or 500MB/s of actual data transfer per lane. So, it is capped at 1Gb/sec recording/reading.

For comparison - top slot uses four v3.0 lines. Single PCIe 3.0 lane at 8GT/s, can send 985MB/s. It is 4 times more maximum possible speed for SSD, good SSD's are close to this numbers in sequential reading speeds.

For this board you have to kill a lot of wishes. Can have one high speed NVMe SSD, but forced to use second low speed NVMe SSD, can add 10Gbit network adapter, but forced to use USB 3.1 Gen2 port on your NAS/server.

Locations of both M.2 slots are good from thermal point of view.

Has Intel network adapter, it is little better with lower CPU load and drivers.

So, we now need to find place for GPU, pair of NVMe SSDs and 10Gbit controller (as on board we have only 1Gbit Realtek).

First SSD goes to M.2 slot - and location of this slot is biggest shortcoming of this board, as it is directly under hot GPU. You need either good heatsink for SSD or can use solutions that use M.2 cable and mount in 2.5" place like some Intel Optane drives.

Second SSD can put into x8 split PCIe second slot and keeps full x4 speed.

So, we now need to find place for GPU, pair of NVMe SSDs and 10Gbit controller (as on board we have only 1Gbit Realtek).

First SSD goes to top M.2 slot - location of this slot is good and it won't be hot as on older board where it is directly under hot GPU.

Second SSD can put into x8 split PCIe second slot and keeps full x4 speed.

Third PCIe 2.0 x4 slot is good for 10Gbit network adapter.

You can also use second M.2 slot for SSD - it is x4 PCIe 2.0 one, not fastest but if your drive is slow it can save you little money on PCIe-M.2 card. Note - second M.2 slot use same lines as lowest of cards slots (PCIe 2.0 x4 one).

If you like to have USB-C Gen2 or USB-A Gen2 connectors you need board with such internal connector.
Otherwise will need to make some custom good elongation cables and it is easy to mess stuff at 10Gbps.

Asus for now is best among companies who support such connectors. Usually it is also top and last boards that have it.

None of board mentioned above has such connector.

Also around 99% of cases lack build in ports with such connector, so you will need to get separate one.